This invention relates to an apparatus for detecting position of a light spot on an optical disk, and more particularly to an apparatus for detecting position of a light spot suitable for a tracking servo control or a track-jumping operation control of an Optical Memory Disk (hereinafter referred to as an OMD).
Hitherto, in the tracking control of an optical disk used exclusively for reproduction such as a compact disk, detection as to whether or not an optical spot for reading information exists on the axis (center line) of the recording track of the optical disk (hereinafter referred to as on-track detection) is conducted generally by using the method called three beam method or the push-pull method.
In accordance with the three beam method, a preceding light spot and a succeeding light spot are emitted at the positions preceding and succeeding to a light spot of an information recording and reading main beam, with predetermined spacings in a direction perpendicular to the axis direction of recording tracks (travelling direction of the light beam), respectively. Rays of reflected light obtained as the result of the fact that the above mentioned preceding and succeeding light spots are reflected on the recording surface of the optical disk are respectively detected by photodetectors to take a difference output between outputs subjected to photoelectric conversion of the detected rays of light.
In this case, if the main light beam exists on the axis of the recording track, the difference output becomes equal to zero. In contrast, if the main light beam deviates from the axis of the recording track in any direction, a difference output of a positive or negative value is produced. In order to allow the value of the difference output to be equal to zero, the positions of the light spots are controlled by employing a method of driving the object lens of the optical pick-up by using an actuator, etc., or the like method. Thus, a tracking servo control is carried out. Further, by counting the number of points at which the value of the difference output are crossing with zero (zero-cross points), the positional control at the time of the track-jumping operation can be conducted.
However, it should be noted that the above-mentioned on-track detection can be carried out by the three beam method only in connection with an optical disk exclusively used for reproduction. In such an optical disk exclusive for reproduction, a quantity of reflected light at the portion of the recording track is smaller than that at the portion subject to mirror finish because pit trains where information signals are stored are formed. Accordingly, difference outputs which are not equal to zero are provided.
However, in the case of OMD of the Write Once Read Many (WORM) type or the Erasable Direct Read After Write (E-DRAW) type, at the non-recorded portion where no information is recorded, there is hardly difference between a quantity of reflected light at the portion of the track on which information is to be recorded (which specifically called a groove in the case of the OMD) and that at the portions except for the above. Accordingly, since there are instances where the difference output becomes equal to substantially zero irrespective of whether or not the light beam is either in the on-track state or in the off-track state, it was difficult to carry out the on-track detection.
The push-pull method is known as the method permitting the on-track detection even in such a case.
In the push-pull method, a bisected or half-split photodetector is used. In accordance with the above-mentioned push-pull method, output terminals of the bisected photodetectors are connected to respective input terminals of a subtracter to take a difference output therebetween. Since the intensities are equal to each other in the case where the light beam is in the on-track state, the value of the difference output becomes equal to zero. In contrast, in the case where the light beam is in the off-track state, the difference output takes a positive or negative value. Thus, the on-track detection can be carried out similarly to the case of the three beam method.
However, even in the case where this push-pull method is used, when the disk is inclined in a radial direction thereof or the optical axis of the lens of the optical system deviates to any degree, an offset is produced in the light detection by the bisected photodetector. When such an offset is produced, even if the light spot exists on the axis of the recording track (groove), a tracking error signal serving as a difference output does not equal to zero. For this reason, an erronerous operation takes place.
In view of the above, the applicants have already proposed a system in which three light beam spots are used to take respective push-pull difference outputs between individual rays of reflected light to carry out calculation of these three push-pull difference outputs to thereby eliminate the influence of the above mentioned offset, thus to permit a satisfactory tracking servo control even at non-recorded portions (Japanese Patent Application No. 283153/90).
However, in the case where waveform shaping is applied to some degree to a tracking error signal generated in accordance with the above mentioned system to feed it back to an actuator, etc. to thereby carry out the tracking servo, if the level of a signal fed back is too low, the drivability for driving the actuator becomes insufficient. For this reason, if there is any great change in the tracking error signal, the actuator cannot follow, resulting in an erronerous operation in the servo operation. In contrast, if the level of a signal fed back is too high, the dribability of the actuator becomes too large. For this reason, a quantity of a signal subject to servo control becomes excessive, so an oscillating phenomenon takes place.
Namely, there exists a suitable or reasonable range in connection with the level of a signal to be fed back. For this reason, it is required to carry out a suitable gain control or adjustment.
When viewed from the above discussion, however, it should be noted that the tracking error signal generated by the above mentioned system (Japanese Patent Application No. 283153/90) becomes equal to a value in the vicinity of substantially zero in the case where the tracking servo control is being conducted (the tracking servo system is in a closed state). For this reason, there was the problem that it is extremely difficult to carry out gain control in the case where the tracking servo is in such a closed state.